TPS is faulty

Well, my check engine light has been on for over a week and Autozone says it's my TPS. They hooked into the OBDII port and the scanner said it was not working correctly. The part from Subaru Parts.com is $156, and I don't mind paying it, but is there a way that I can fix my existing one? I've read some of the cleaning threads, but I'm not sure if that was to "improve" a good one, or to fix a bad one. The car runs, and I havn;t really noticed anything out of the ordinary, but I want to fix it. Oh, I did notice a drop in MPG (about 3-4 MPG) over the past few weeks. Thanks in advance for the info.

Clarke

[Landshark]

it basically just a potentiometer, or variable resistor. the resistance changes as the throttle is depressed and a wiper arm moves across the contact. its probably worn out, but you could try cleaning it with some electronic contact or TV tuner cleaner available at Radio Shack.

So, is it something that I can simply replace, or does it require re-tuning the engine after replacement? Thanks.

Clarke

[Landshark]

just carefully scribe some marks on the old one before you remove it so the new one can be aligned correctly. don't use a pencil! (like i did) - SCRATCH the marks into the TPS and throttlebody......and be careful not to drop the little bolts. you can pop off the negative battery cable before you do it so the computer resets, or just let the computer adapt to the new sensor (will take longer, i guess.)

[lee]

if you have a volt-ohm meter (VOM), there's a post by Trevor (do a search) that describes a very easy method to setting the TPS. It can be done before you even start the car after the change.

So, all you need is a phillips screwdriver, a VOM and it's done in about 5 minutes.

So, should I just assume the TPS is shot, or is there something I should do to verify that it's faulty. I do not notice anything wrong with the drivability of the car so I am reluctant to drop $150 on a part like this. Should I reset the Check Engine light and see if it comes back? Should I remove the TPS and try to clean it? Thanks in advance.

[lee]

if there are no driveability problems, then yes I would reset the ECU and see what happens.

OK, I removed power last night and the check engine light is now off. If the TPS is faulty, how long should it take for the light to come back? Thanks in advance.

[lee]

I don't really know, but I seem to remember Beav saying something about two drive cycles (not just turning on off twice, but actually cold to warm driving time cycles). However, there's a better than average change I misunderstood what he said.

[immortal_suby]

Clark - before you drop 150 on a tps I'd be more than happy to swap tps with you to diagnose to make sure you need to spend it. I have a new tps in my car.

I'd give yours a week and if the CE light doesn't come on you're probably in the clear.

Thanks again Suby... Hopefully I'll not need to change it out. Thanks.

Clarke

[Beav]

A TPS will go bad normally at the 'just off idle' position, where they see the most activity (read: motion.) Sometimes you can 'see' a glitch if you use an ohm meter, but the best diagnosis is with a graphing multi-meter (kind of like an oscilloscope.)

The fact that the computer set a code for the TPS is not truly indicative that the TPS is actually bad, it could be the wiring or the connection at the TPS or the computer. This is true of most codes set by computers - they just 'know' that something is not right on a particular circuit. I'm going to be a bit verbose here (is this a first for me? heheh) in this explanation, as it seems to be a source of confusion for many. Let's say that an OBDII car sets a code of P0171 - the upstream O2 sensor on bank #1 is reporting too lean and the fuel system has adjusted itself to full rich, yet the sensor is still reporting too lean. Autozone will say "Hmmm, it says the O2 sensor is only showing lean" and will sell you an O2 sensor. However a broken vacuum hose will cause the same code, as will a restricted fuel filter, a partially plugged exhaust, a bad fuel injector(s), bad fuel regulator, yadda, yadda, yadda. Whaddya think the chances are that they'll take the sensor back after it has been installed? Will they jack up your car and put your original sensor back on for you? Will they say "Ooops, we were wrong" ? And what about that beer you spilled when you were crawling out from under your car?

Now I'm going to back up just a little bit and say that most TPS codes end up being a bad TPS, but it could still be something else. The question becomes "How much are you willing to gamble?" You could risk the buck and a half and probably come out O.K. Or you could pay a shop about an hour's labor for a diagnosis and possibly find out what you already suspect. They may confirm your suspicion, replace the part and find out they were wrong. Then they are out the time and money, not you. Then again they may find a bad connection right away and save you some bucks. What I'm getting at is that it is not always cut and dried, that a scanner is not the end-all tool many consumers think it is.

Regarding the drive-cycle issue: Some problems need to occur more than once to set a code - it depends on the make, model, engine, programming, seriousness of the malfunction, etc. This is to keep small, unforseen glitches in programming from causing unnecessary trips to the service bay (I wish they'd make a light that detected a loose/missing gas cap before setting an error code.) Some serious errors will set a code if the error is static for only one second (just had one today - MAF sensor on a Lumina APV.) Some errors must occur over several consecutive drive cycles. Some errors will be recorded, yet never turn on the MIL (malfunction indicator lamp.) Flip-side: if an error doesn't recur in 40 or 50 consecutive drive cycles the code may be cleared by the computer and the MIL turned off (generally on OBDII vehicles only.) I don't know about the rest of you but I don't think I could go 40 or 50 start-ups without finding out what caused the 'Check Engine' light to come on.